On Fri, Aug 19, 2022 at 05:21:58AM +0000, Arseniy Krasnov wrote: > Hello, > > This patchset includes some updates for SO_RCVLOWAT: > > 1) af_vsock: > During my experiments with zerocopy receive, i found, that in some > cases, poll() implementation violates POSIX: when socket has non- > default SO_RCVLOWAT(e.g. not 1), poll() will always set POLLIN and > POLLRDNORM bits in 'revents' even number of bytes available to read > on socket is smaller than SO_RCVLOWAT value. In this case,user sees > POLLIN flag and then tries to read data(for example using 'read()' > call), but read call will be blocked, because SO_RCVLOWAT logic is > supported in dequeue loop in af_vsock.c. But the same time, POSIX > requires that: > > "POLLIN Data other than high-priority data may be read without > blocking. > POLLRDNORM Normal data may be read without blocking." > > See https://www.open-std.org/jtc1/sc22/open/n4217.pdf, page 293. > > So, we have, that poll() syscall returns POLLIN, but read call will > be blocked. > > Also in man page socket(7) i found that: > > "Since Linux 2.6.28, select(2), poll(2), and epoll(7) indicate a > socket as readable only if at least SO_RCVLOWAT bytes are available." > > I checked TCP callback for poll()(net/ipv4/tcp.c, tcp_poll()), it > uses SO_RCVLOWAT value to set POLLIN bit, also i've tested TCP with > this case for TCP socket, it works as POSIX required. > > I've added some fixes to af_vsock.c and virtio_transport_common.c, > test is also implemented. > > 2) virtio/vsock: > It adds some optimization to wake ups, when new data arrived. Now, > SO_RCVLOWAT is considered before wake up sleepers who wait new data. > There is no sense, to kick waiter, when number of available bytes > in socket's queue < SO_RCVLOWAT, because if we wake up reader in > this case, it will wait for SO_RCVLOWAT data anyway during dequeue, > or in poll() case, POLLIN/POLLRDNORM bits won't be set, so such > exit from poll() will be "spurious". This logic is also used in TCP > sockets. > > 3) vmci/vsock: > Same as 2), but i'm not sure about this changes. Will be very good, > to get comments from someone who knows this code. > > 4) Hyper-V: > As Dexuan Cui mentioned, for Hyper-V transport it is difficult to > support SO_RCVLOWAT, so he suggested to disable this feature for > Hyper-V. > > Thank You Hi Arseniy, Stefano will be online again on Monday. I suggest we wait for him to review this series. If it's urgent, please let me know and I'll take a look. Thanks, Stefan > Arseniy Krasnov(9): > vsock: SO_RCVLOWAT transport set callback > hv_sock: disable SO_RCVLOWAT support > virtio/vsock: use 'target' in notify_poll_in callback > vmci/vsock: use 'target' in notify_poll_in callback > vsock: pass sock_rcvlowat to notify_poll_in as target > vsock: add API call for data ready > virtio/vsock: check SO_RCVLOWAT before wake up reader > vmci/vsock: check SO_RCVLOWAT before wake up reader > vsock_test: POLLIN + SO_RCVLOWAT test > > include/net/af_vsock.h | 2 + > net/vmw_vsock/af_vsock.c | 33 +++++++- > net/vmw_vsock/hyperv_transport.c | 7 ++ > net/vmw_vsock/virtio_transport_common.c | 7 +- > net/vmw_vsock/vmci_transport_notify.c | 10 +-- > net/vmw_vsock/vmci_transport_notify_qstate.c | 12 +-- > tools/testing/vsock/vsock_test.c | 108 +++++++++++++++++++++++++++ > 7 files changed, 162 insertions(+), 17 deletions(-) > > Changelog: > > v1 -> v2: > 1) Patches for VMCI transport(same as for virtio-vsock). > 2) Patches for Hyper-V transport(disabling SO_RCVLOWAT setting). > 3) Waiting logic in test was updated(sleep() -> poll()). > > v2 -> v3: > 1) Patches were reordered. > 2) Commit message updated in 0005. > 3) Check 'transport' pointer in 0001 for NULL. > > v3 -> v4: > 1) vsock_set_rcvlowat() logic changed. Previous version required > assigned transport and always called its 'set_rcvlowat' callback > (if present). Now, assignment is not needed. > 2) 0003,0004,0005,0006,0007,0008 - commit messages updated. > 3) 0009 - small refactoring and style fixes. > 4) RFC tag was removed. > > -- > 2.25.1
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